To see what would break, I also ripped out a whole lot of other stuff on it as well:
openrc and everything in /etc/init.d/ /etc/conf.d/
syslog-ng and its configuration
logrotate and its configuration
vixie-cron and its configuration and /etc/cron.*/*
sysvinit and /etc/inittab
xinetd and /etc/xinet.d

What breaks:

1. There are some ebuilds that make use of some general-purpose functions exposed by OpenRC. There is a bug talking about what to do about this, but nobody has done anything. So, can either:
a) leave openrc installed for the time being (and the symlink within /etc/init.d named "functions.sh") <--- probably the smart way to go for now
b) remove openrc, but replace the functions.sh link with a shell that provides the necessary functions (if you hunt around for "systemd-only gentoo", you can find one)

2. systemd is written in such a way (and packaged by the Gentoo maintainer in such a way that) that it can co-exist with all of the things above that I removed. The default install of systemd seems to assume that you do still have some kind of syslog daemon (to which it will copy all its logging output), and that you have left sysvinit in place for backward-compatibility (so that you can still use sysvinit commands such as "reboot" and "init"). Neither of these are essential, but they do seem to be the default, probably for a good reason. The only consequences I have observed of having removed them are:
a) I had to read the systemd man page to learn some equivalent commands (e.g., on that machine, I now type "systemctl reboot" instead of "reboot")
b) There are a couple of services and targets associated with backward compatibility with sysvinit and associated with syslog, that attempt to start and fail. These don't cause any delays or problems, so I haven't done anything about it yet.
I would probably suggest leaving sysvinit in place, but getting rid of syslog (if you don't have any compelling reason to keep it).

Don't go just by what I've typed here, which is not a complete "how to". Several other people have offered advice about how to go about this, if you search._________________History teaches us that men and nations behave wisely once they have exhausted all other alternatives. -- Abba Eban

Yeah, you can get a replacement 'functions.sh' from that second repository._________________History teaches us that men and nations behave wisely once they have exhausted all other alternatives. -- Abba Eban